You should establish your key early in the lay-in. "Keying" a painting means deciding where on the value scale your lights and darks will fall. A painting might run from an inky dark to a brilliant light, or the whole painting might be high key, that is, few or no darks. The whole painting takes place at the top of the value scale. A low key painting contains only lower (darker) values and few light ones. How a painting is keyed is your choice. You can choose to match the notes of nature in front of your easel or you can transpose up or down to a higher or lower key. Often a painter keys a landscape by spotting a few of the darkest darks and the lightest lights. Where the horizon meets the sky is a big contrast and if recorded early will set up the key for a painting.Your key might include only the top end of the value scale, the middle or the bottom or the entire range of values.
The painting above is in a low key. The darks are strong and rich and the values of the water and sky are only high in comparison to those darks. If you isolated a spot from the water or sky on a white field they would be surprisingly dark.
|John Singer Sargent|
A high key gives a look of brilliant light and an ethereal shimmering feminine quality. A low key gives a look of power and drama and a masculine quality. In a high key painting the color notes lose a lot of their chroma because they are so mixed with white. High key paintings are probably more saleable. The old masters generally painted in a low key .
In a low key painting the notes are full of rich color as there is more colored pigment in each note than white. Just as transposing a tune down an octave on the piano gives it more richness and color, transposing a passage in color down an octave gives a richer deeper color.
If you paint on a dark ground you will often inadvertently end up with a lower key. Painting in a high key is a lot easier on a white canvas. When high key color is weak it is chalky. When low key color is weak it is murky and "too dark".
Of course a painting can also run the gamut, that is, it can be keyed not to a section of the value scale, but to the entire value scale and have all of the values from ink to pure white on the canvas. That is probably the most common key in practice. You get a lot of those on sunny days. It is nice to know how to raise the key of a painting a little when you are out on a gray or dark day. Keying up a rainy day or gray day picture can save it from being lugubrious.